In magnetic disk drives, data is written and read by magnetic transducers called “heads.” The magnetic disks are rotated at high speeds, producing a thin layer of air called an air bearing surface (ABS). The read and write heads are supported over the rotating disk by the ABS, where they either induce or detect flux on the magnetic disk, thereby either writing or reading data. Layered thin film structures are typically used in the manufacture of read and write heads. In write heads, thin film structures provide high areal density, which is the amount of data stored per unit of disk surface area, and in read heads they provide high resolution.
A thin film write head may have two pole pieces, namely, a top pole piece (colloquially referred to as “P2”) and a bottom pole piece (“P1”). A write head generally has two regions, denoted a pole tip region and a back region. The pole pieces are formed from thin magnetic material films and converge in the pole tip region at a magnetic recording gap, and in the back region at a back gap. A write head also has two pole tips, sometimes denoted “P1T” and “P2T”, that are associated with and that are extensions of the poles P1 and P2, respectively. The pole tips, which are relatively defined in their shape and size in contrast to the pole pieces, are separated from each other by a thin layer of non-magnetic material such as alumina or Rhodium, referred to as a gap. As a magnetic disk is spinning beneath a write head, the P2 pole tip P2T trails the P1 pole tip P1T and is therefore the last to induce flux on the disk. Thus, the P2T dimension predominantly defines the write track width of the write head, and is generally considered an important feature.
The write track width, which is related to the width “P2B” of the bottom of the pole P2, is especially important because it limits the areal density of a magnetic disk. A narrower track width translates to greater tracks per inch (TPI) written on the disk, which in turn translates to greater areal density. However, with present manufacturing methods for read-write heads, the ability to produce very narrow track widths is limited. These limitations will be further explained with reference to a specific type of inductive head. Inductive heads commonly employed at present are magnetoresistive (MR) sensors, which are highly sensitive to changes in magnetic flux on a disk written by inductive write heads. An MR sensor includes a thin film layer sandwiched between bottom and top insulation layers, or gaps, which are in turn sandwiched between bottom and top shield layers, S1 and S2. An MR head can read information on a magnetic disk with much narrower track widths an much higher fidelity than other known types of read heads. The apparent ability of MR sensors to read very narrow track widths may enable the use of narrow track width write heads and therefore lead to high areal densities. While this advantage has been sought through the use of photoresist frame plating and ion beam milling of write heads, manufacturing heads with very narrow track widths remains a significant challenge.
A particular type of MR head is a so-called “piggy back” MR head. A “piggy back” MR head has a shield S2 and a bottom pole P1 of the write head. While such MR heads have a high capacity for both reading and writing, they are limited in the narrowness of the track width they may utilize because they have been found to possess large side-fringing fields during recording. These fields are caused by differences in pole tip (P1T and P2T) widths. The fringing field, caused by flux leakage from the second pole P2 to the first pole P1 beyond the width of the second pole P2, is the portion of the magnetic field which extends toward the tracks adjacent to the tracks being written. The fringing fields require lower pole tip TP1 in order not to impinge adjacent tracks, thereby limiting the achievable areal density.
The present invention thus understands that current methods for making write heads result in relatively high track width “sigmas”, or differences between track widths wafer to wafer in the manufacturing process. Also, the present invention understands that current methods can result in undesirably high differences between the widths “P1A” and “P2B” of the poles tips P1T, P2T and in undesirably high differences between the wall angle tapers of the poles P1, P2.